I remember towards the end of wotlk, I spec’d prot/holy on my pala, combining ret relentless gear with holy relentless gear to get double 100 resilience bonus and get as much spellpower as possible through the deep prot talent+spellpwr from gear and getting glyph of divine plea, salvation and righteous fury for max defense.
To sum it up, the enemy would have to damage me through sacred shield, 64% armor from plate&shield, 24% res, 2400 spellpower (which ofc empowers sacred shield), blessing of sanctuary(3% dmg reduction was it?), glyphs (salvation 20% reduction when enabled)+ endless mana with divine plea renewing every melee hit and strong holy light heals.
Tried it yesterday, and people could max crit me for 3k, usually 2k.
I love this spec.